This invention relates to a method for amplifying multiplexed analog signals of large dynamic range, i.e., whose level is liable to large variations, and a device for carrying out said method.
The amplification method is particularly adapted to the amplification of analog signals issued from geophones, arranged on the earth surface, in response to seismic shocks transmitted through a ground layer being surveyed and reflected by the reflecting layers or mirrors of the sub-soil.
According to a known process, the signals issued from each geophone are amplified by a preamplifier and then by a variable gain amplifier made up of a cascade arrangement of amplification units connected in series, each unit having a fixed gain preferably equal to an n.sup.th power of 2, n being an integer (binary gain amplification system). A selector member, detecting the average value of the amplitude of the signal issued from the series of amplification units, serially connects an increased number of amplification units to compensate for the progressive decrease of the input signals versus time. The amplified signals are then applied to a chain comprising a multiplexer, an analog-to-digital converter and a digital recorder.
Such a system is described, for example, in the U.S. patent application Ser. No. 99,808.
According to another known process, the seismic signals issued from each geophone or group of geophones are directly supplied to the inputs of an analog multiplexer, after an optional preamplification in a fixed gain preamplifier. The output of the multiplexer, where there are available a sequence of signals, is connected to a single amplifier which is also made up of a chain of amplifying elements arranged in series, the output of each element being connected to the input of the following one. The gain of each amplifying element is fixed and preferably chosen as an n.sup.th power of 2, n being an integer.
Since the amplitude from one sample to the next can vary substantially, a selector member selects the number of amplification units through which each signal must pass to reach an optimum level, and connects the output of the amplification unit where this signal is available to the input of an analog-to-digital converter which is connected to a recording system. Such a process is described, for example in the U.S. Pat. No. 3,264,574.
The amplifiers, formed of a series of amplifying units connected in series suffers from certain drawbacks.
As a matter of fact they are relatively slow in view of the cumulative character of the delays affecting the signals when passing through the different amplification units. Similarly the effects of the relatively slow response of each amplification unit, when operated with pulses, characterized by its slew rate, cumulated over all the serially connected elements of the chain, contribute to reduce the overall rapidity of the amplifier response.
There can be also mentioned the overflow of the amplification units which is liable to occur when a sample of low amplitude is followed with another sample of substantially higher value.
A disadvantage inherent in data acquisition chains comprising a head multiplexer further consists in the fact that, for memorizing the sample value during the gain selection step, there must be used a memorization element, currently called sample and hold unit, connected before the amplifier and, consequently, operating on signals of wide amplitude range. As recalled in the following description, a sample and hold unit is technologically more difficult to operate with small signals.